Friday, June 3, 2011

a hammer-throw type surface based spinning tether lunar launcher

As a way to get stuff off the Moon, how about a hammer-throw type surface based spinning tether lunar launcher (suggested by "Robert" at Selenian Boondocks about a year ago), a 200km radius of rotation gets 50 tonne manned ship to escape velocity
with a centripetal acceleration of 28m/s (2.8g), rotation speed of 6.8 rpH, at 50GPa strength the tether weighs only about 15 tonnes.

To decrease the time between launches, you could have the
payloads travel down a permanently extended tether.
When you get to this radius of rotation, the possibility exists for the tether to catch
payloads from space, making low g rocket free transport available in
both directions.

Using the mass driver you face a major problem in power storage and
rate of discharge, the solution is usually to launch small
payloads, but using the "hammer-throw type surface based spinning tether
lunar launcher" the power storage issue disappears, the tethers rate of
rotation can be increased as slowly as power supply allows, and the high
payload/ tether mass ratio allows large individual payloads.

A tether with a radius of 4km located at Earth-Moon L2 would catch payloads arriving at
270m/s, subjecting them to a centripetal acceleration of 18ms^2, tether
rotation is at 3.86 degrees/sec. A carbon nanotube tether for a 50 tonne
payload/manned ship need only weigh about 200kg(!) at 50GPa strength,
so would need to rotate around something substantial, EML2, as you probably know,
is unstable, so the momentum of the arriving ship could probably be
used to maintain the receiver at EML2 if positioning at capture was
done with that in mind.

Energy costs for the tether option at 10c/KWhr are about $75/tonne assuming negligable losses, perhaps $100/tonne including losses.

Update 8th June: Well this idea has been looked at by the experts after all, it's covered towards the end of this document.


  1. Hello Andrew,

    I'm following up from your comments on the QuantumG blog. I made a small spreadsheet on the ground level rotovator and an illustration of the concept at my website. Is this what you where thinking of? I think the concept lends itself best to a small implementation, using for launching people seems a stretch. However, a series of these things could be built for greater capacity. I've been thinking of using 600g acceleration, but it may be a bit extreme for the payload and especialy the hook.
    I read the link you provided (very interesting document,by the way)and i'm wondering these days about the production facilities required to supply the rotovator with useful materials. Do you have any info on this? And what are your thought on a small rotovator?

    Michel Lamontagne

  2. Hi Michel, thanks for your comment.

    I think you're probably right about a small and lighter surface based launcher at least initially. The big hurdle is going to be just getting started, everything needs to be done on a business basis. What would see a move towards a bigger low g launcher is lots of people to shift.

    I'm not sure what you mean by "production facilities required to supply the rotovator with useful materials". Do you mean the payload to be launched and methods of extracting H2O as rocket propellant, or something more sophisticated like platinum mining? I'm a fan of the types of space colonies advocated by Gerard O'Neill in the '70's, and with their system the payloads were just Lunar regolith to be refined in space with the slag or tailings serving as simple mass based radiation and meteor shielding.

  3. I had a look at your spreadsheet, it's enlightening and yeah, taking into account the materials that we actually have now it interesting to see you still get a good return in payload for the mass invested. The weight of the solar cells is a worry, but you maybe a bit pessimistic on that, see my latest post which has a link claiming 1kg/KW for space based power. It did occur to me that the rotovator is itself an efficient energy storage system, so possibly you could throw larger less frequent loads by having a set as energy storage flywheels powering a launcher.

    I imagine these launchers could be based near the lunar poles to get the solar cells into near continual sunlight.

  4. Hello Andrew,

    Since you’re interested in ONeil space colonies, I hope you will enjoy the spreadsheet I’ve added to my website, an ONeil cylinder calculator. Playing with the numbers, I’ve come to the realization that the main obstacle to space colonization is probablythe cost of the first colony. That is why I like the idea of the moon surface rotovator. Very high output to weight ratio and a ‘low’ first cost. If the solar cells can be lighter, so much the better, but I expect surface mounted cells, even in a low G field, must be stronger and therefore heavier than in space. The radiators and cooling circuits required will also add to the overall cost.

    What I’m wondering about now is how small can effective moon production facilities be made? How big is a 5000T per year lunacrete factory, or an oxygen/hydrogen production facility? How big is an aluminum container making plant, for the fuel storage?

    On another tack, I’ve seen an important change over the last few years in how development is done here on Earth, particularly in Quebec where I live, but, I’ve read, in Australia and other places as well. It used to be: a new mine = a new city. But now, workers are flown in on shifts, live on site in minimal dormitories and return back to their homes in the south. Mine owners do not want to be responsible for a city: It’s not their ‘core business’.
    So; if launch costs are low, say 50$/kg, and solar power is 1kg/kW, and power 0,1$/kwh hour, 8000 hours of operation will generate 800$ of revenue per year per kg. There is then no need for space production facilities! Just for maintenance and assembly, than can be done by people on shifts, since an 80 kg person will ‘cost’ only 4000$ to bring up. (lots and lots of rounding off here, I realize!)
    Space shacks rather than space colonies.

    Any thought on this? Does the O’Neil colony paradigm still hold?


    Michel Lamontagne

  5. Certainly a solar array on the Moon will be heavier than one in space, but if mounted at one of the poles I don't think that much heavier. You'd just hang it from a rotating vertical mast, without wind loadings to worry about such a mast hundreds of feet high could support several times its own weight, I don't see cooling being more of a problem than in deep space.

    I'm not qualified to judge how much mass in the form of processing plant would be required for each tonne of product, In T. A. Heppenheimer's book Colonies In Space there is discussion in chapter 7 on the processing plant proposed for the O'Neill colonies: "Driggers proposes that there be two construction shacks, one to do the extraction of metals, the other to do the actual construction"..."The two shacks together weigh 7000 tons, with another 3000 tons for the power plant".

    If the costs of getting into space come down in the way I think is possible, whether people make their homes in space or commute from Earth in the way you suggest I think will come down to choice, personally I like the scenario of living in a colony drawn along the lines that are suggested in Heppenheimer's book, but if the "colony" were a claustrophobic space station, the "homes" being tiny boxes, I'd choose to live on Earth and commute.

    Maybe the move away from establishing mining towns to commuting could mainly be attributable to people preferring to live in the established city with all it's amenities, rather than live in a little hick town where there's nothing exciting to do on a Saturday night?